Thermodynamic System Lab Report

INTRODUCTION:

Phase:

A thermodynamic system having throughout uniform physical properties and composition is described as a phase. Ice, liquid water and water vapours are three phases in which water exists. For a multiphase system each phase shows certain macroscopic properties very different from the other phases. The three phases of water, for example, can be distinguished by very different densities. These three phases exemplified above for water constitute three distinct physical states of water i.e solid, liquid and gas. Different phases may also be present in same physical state. For example, a vessel containing immiscible oil and water are present in liquid state and constitute two different phases as the densities of phases are different.

This phenomenon is termed as phase transition. Phase transitions can be exemplified as: Transitions involving change in state of matter, i.e. solid, liquid and gas. In these transitions different phases differ with respect to their densities. Transition involving superconductive behavior of certain metals on cooling below critical temperature. The metals show nonzero resistivity at temperatures above critical temperature while their conductivity drops to zero below critical temperature. Paramagnetic and ferromagnetic transitions in magnetic materials. Above Curie temperature, the paramagnetic phase contains randomly oriented magnetic moments in the absence of external magnetic field and shows no net magnetic moment. However below Curie temperature randomly oriented magnetic moments align together and generate a net magnetic moment in the absence of external magnetic

Non centrosymmetry is main requirement to measure these effects at molecular as well as bulk level. SHG does not occur due to isotopic nature of solution containing NLO active molecules. However dipolar molecules become aligned by application of static electric field in EFISHG (Levine & Bethea, 1975; Meredith, 1982; Singer & Garito, 1981) experiment. In this way centrosymmetry is broken and coherently doubled frequency light is detected on passage of probe beam through sample. Application of static electric field is ruled out in HRS (Clays & Persoons, 1991; Clays, Persoons, & De Maeyer, 1994) technique as it relies on local anisotropy of solution to break bulk centrosymmetry. Advantage of HRS over EFISHG is the detection of 2nd order response of molecule lacking permanent dipole (octopolar molecules) and ions. Detection of incoherent doubled frequency scattered light is disadvantage of HRS as compared to